1. Technical Field
The invention relates to techniques for controlling transmission of multicast packets over a local area network, and was developed with specific attention paid to the possible application to Wireless LAN (WLAN).
2. Description of the Related Art
A number of solutions related to multicast and wireless communications explicitly describe techniques to improve reception quality when a plurality of receivers share the same wireless channel.
Service providers offering “triple play” (data, video and voice) services usually add Internet Protocol multicast support to their routers. Services like video on demand do in fact benefit from Internet Protocol multicast arrangements, because the overall network traffic can be dramatically reduced in comparison with the unicast case.
Wireless LAN (WLAN) technology is being increasingly used in order to deliver data, video and voice in the home environment. Wireless LANs have appeared in the domestic environment mainly for sharing access to a broadband Internet connection. IEEE 802.11b running at 11 Mbps is widely used nowadays, while higher speed IEEE 802.11g at 54 Mbps is appearing.
Consumer Electronics (CE) manufacturers are interested in using a domestic wireless LAN to distribute audiovisual content among entertainment devices and Personal Computers. However, variations in the wireless link conditions in the home environment—due to propagation effects, interference, and traffic generated by other devices—cause transmission errors and packet losses. Additionally, packet transmission over wireless LAN suffers from signal attenuation as caused by walls and multipath effects in a closed environment like the home. Furthermore, as wireless equipment in the 2.4 GHz ISM band is becoming commonplace, multiple users may be sharing the same radio spectrum in an uncoordinated way, thereby producing mutual interference.
As indicated, the variations in a wireless link in a home environment, primarily due to propagation effects, interference, and traffic generated by other devices, cause packet loss, especially when using multicast transmissions. This is due to the lossy characteristics of the wireless channel, which makes it almost impossible to transmit a row of pieces of data and receive them correctly on the receiver side. In the Media Access Control (MAC) layer of IEEE 802.11, the Automatic Retransmission Request (ARQ) and the Dynamic Rate Shifting (DRS) are employed to improve channel reliability. In case of transmission errors on unicast packets, the Automatic Retransmission Request feature of the MAC layer will trigger multiple retransmissions of any erroneous packet either until this is correctly received or for a given number of times. When the channel condition becomes really poor, the Dynamic Rate Shifting feature will reduce the physical layer transmission rate in order to boost up the amount of energy transmitted per symbol.
When transmitting multicast packets over a WLAN, packet loss is more probable than in the case of unicast packets (with the same size and the same physical rate) because receiving stations cannot send back 802.11 Acknowledgments to the Access Point (AP). The 802.11 standard does in fact forbid Acknowledgment messages in the case of multicast frames. This means that multicast frames cannot be retransmitted, while unicast frames can be retransmitted.
The reason why Acknowledgments are disabled for multicast transmissions lies in the need of avoiding Acknowledgment “implosion” at the Access Point. Moreover it would be quite difficult for the Access Point to retransmit frames according to the multiple error patterns experienced by different receivers.
Thus a feasible way to reduce packet loss for multicast packets is reducing the physical rate during multicast transmissions. Lowering the physical rate can effectively increase robustness, but it causes a degradation of the aggregated throughput that can be achieved on the WLAN (bandwidth waste).
Document US-A-2002/028687 does in fact describe a retransmission control method and system for multicast information distribution service. This document describes a retransmission control method for a multicast information distribution service that distributes multicast information with respect to a plurality of wireless terminals within a service area from an information distribution apparatus via a wireless region. In particular, in a retransmission control method for a multicast information distribution service, when information, which requires retransmission, is generated, a wireless terminal transmits the retransmission request for the information to an information distribution apparatus when a timing determined for the wireless terminal is reached. The information distribution apparatus which receives the retransmission request transmits retransmission information which indicates the information requested by the retransmission request, with respect to each of the wireless terminals within the service area, and retransmits the information requested by the retransmission request at a predetermined timing.
Document US-B-6 522 650 describes a multicast and broadcast transmission with partial Automatic Retransmission Request (ARQ) in a network of devices connected to a shared transmission medium, wherein a frame is to be transmitted to a plurality of the devices. To support the partial Automatic Retransmission Request, a Media Access Control (MAC) protocol uses robustly transmitted frame delimiters and a Media Access Control management frame structure. In a multicast or broadcast frame with partial Automatic Retransmission Request, a destination address field is set to that of a selected proxy device address, and the address of the intended recipients (of group address of the intended recipients) is placed in an entry in the Media Access Control management frame structure. The frame also contains a multicast flag set to indicate that the transmission is intended for more than one device and a value in the frame delimiters to indicate that a response is requested. Upon receipt of such a frame, the proxy device specified by the destination address field provides an appropriate response type on behalf of the group. The non-proxy recipients, detecting the multicast flag setting, know to ignore the Destination Address (DA) in the destination address field and instead look to the Media Access Control management entry for actual destination addresses.
Document US-B-6 671 284 relates to a robust Media Access Control (MAC) scheme for network stations operating in an Orthogonal Frequency Division Multiplexing (OFDM) transmissions network. The Media Access Control scheme uses robustly transmitted frame control information to ensure network synchronization (for media access) and convey channel access prioritization for Quality of Service (QoS). Frame control information can occur in a frame before and after the frame payload, or in a response.
Document JP 2003 023428 relates to multicast reception station, multicast transmission station and wireless multicast communication method. The relative arrangements can suppress the interference quantity of an incoming channel to the utmost and have proper data transfer efficiency, even under-poor channel quality. The wireless multicast communication method includes a number of steps of transmission Acknowledgement (ACK), through the use of an individual physical channel from a reception station, in response to multicast data with a poll request sent through a common physical channel and a step of selecting one reception station by a transmission station for an individual poll notice, on the basis of a value Last Sequence Number (LSN) included in the acknowledgement notice and permitting retransmission request for the multicast data, to only the selected reception stations. This document specifies use of an acknowledgement (ACK) in response to multicast data with a poll request.
Document TW 453 057 relates to delegate mobile host in non-backbone wireless mobile networks. A delegate mobile host mechanism is described to achieve point-to-point transmission and multicast in a non-backbone wireless mobile network (also called “ad hoc network”). A delegate mobile host is an agent that sends and receives packets for mobile hosts in its transmission range. This reduces routing and multicast overhead in the system. When a mobile host sends packets, it sends them first to its delegate mobile host. Then the delegate mobile host transmits in turn toward the receiver. If the mobile host wants to receive packets, its delegate mobile host can receive packets for it to be passed on. The connection between the delegate mobile host and a backbone based wireless network takes a base station as a delegate mobile host and set up a connection by the methods described in the foregoing. The delegate mobile host can also be configured to be an agent of multicast subscribers in order to decrease the redundant packets.
In document US-A-2002/0184389 an 802.11 source station transmits a signal with a duration field other than the one required for the transmission. This is done with the aim of preventing transmission by other stations during known sequences. Thus, the source station uses the duration field to spoof the actual time the medium will be occupied with respect to stations within range of the signal. A station within range of the transmitted signal will check the duration field of the transmitted signal, and update the station network allocation vector. Thus, the station will not transmit because the station network allocation vector indicates that the medium is in use, even though the station may be unable to sense the carrier. Accordingly, spoofed stations may, for example:
1) delay transmission until a more critical transmission has completed,
2) allow unknown or foreign protocol to have preferential use of the medium,
3) prevent interference from hidden stations, and
4) allow sharing of the medium by overlapping basic service sets.